When a vehicle is at rest with the engine operating and the transmission in forward drive, it is desirable to release a fluid operated friction device in the transmission to a slipping condition. In this condition, the friction device transmits minimum torque such that the torque converter is not required to operate in a stall condition. These systems increase the overall efficiency of the powertrain by improving fuel economy. This condition has been denoted as neutral idle. It is important that neutral idle be transparent to the operator by minimizing the disturbance associated with the re-engagement of the friction device. In many prior art devices, the fill time of the apply clutch of the friction device cause an undesirable shift disturbance in the transmission.
Other prior art systems use an automatic neutral control wherein the forward clutch is released when the throttle is released, the brakes applied and the vehicle is at rest. These controls then automatically shift back to drive when the vehicle brake is released. These systems fully release the torque converter turbine from the planetary gear set and in doing so, improve the vehicle fuel economy. However, the clutch fill time associated with these devices and the ability of the operator to increase the input torque with throttle advance can result in an undesirable shift quality and heat generation in the clutch.
Auto neutral control have generally en compass ed one of the following control methods:
1. The partial fill method which tries to keep the clutch apply chamber filled without enforcing clutch disc contact. This is usually accomplished by con trolling the clutch apply pressure through three successive ramps followed by holding the clutch pressure at a constant level. While this method reduces clutch fill time, it produces inconsistent clutch apply times due to the variable holding pressure and leakage at the clutch piston.
2. The torque converter slip method which maintains the clutch filled by controlling the torque converter slip at a very low level. An advantage of this method is substantially constant fill time. A disadvantage is the possibility of heat generation at the clutch during long periods of continuous slip.
3. The intermittent slip method described in U.S. Pat. No. 5,803,869 issued to Jamzadeh et al on Sep. 8, 1998. This method continuously engages the friction device to a torque level less than full engagement and then disengages the friction device to a non-slip condition such that the friction alternately slips and runs free. While this method appears to be very effective, a significant amount of computer programming is required for its operation.